This invention relates to color projection television (PTV) display devices using monochrome cathode ray tubes (CRTs), and more particularly relates to such devices using such tubes incorporating interference filters on the inner surface of the display window.
Monochrome CRTs for color PTV each employ a single electron gun mounted in the neck of the tube to focus a single electron beam on the fluorescent display screen of the tube. A deflection yoke surrounding the neck of the tube, and associated electronic circuitry, cause the beam to scan the screen as well as to vary in intensity in response to a video signal to produce a monochrome display image.
In color PTV, three such displays, each in one of the primary colors red, blue and green, are superimposed on a large projection screen to produce a full color display image. Because the images on the individual tube screens are not viewed directly, but are magnified and projected by a system of projection lenses, the individual cathode ray tubes are driven at higher loads than would be encountered for direct view tubes, in order to produce a full color display of acceptable brightness.
Projection tubes having a multilayer interference filter on the display window are described in U.S. Pat. No. 4,634,926, assigned to U.S. Philips Corporation. The filter, herein referred to as a shortwave pass (SWP) filter, is composed of alternating layers of materials of high and low refractive index. The filter is designed to result in a marked increase in luminous efficiently of the tube in the forward direction, as well as improved chromaticity and contrast. Even further improvements are provided, especially in light gain in the corners of the display screen, by combining such an interference filter with an inwardly or convexly curved display window, as provided in U.S. Pat. No. 4,683,398, also assigned to U.S. Philips Corporation.
In these tubes, the interference filter is deposited directly upon the inner surface of the display window, and the luminescent phosphor screen is deposited on the interference filter. The filter is typically composed of from 14 to 20 layers, each having a thickness of about one quarter of the central wavelength of the filter.
Such tubes with interference filters, while exhibiting a marked increase in luminous efficiency in the forward direction, as well as improved chromaticity and contrast, also exhibit greater luminance in the center than at the edges of the display, sometimes referred to herein as center-to-edge luminance gradient.
This center-to-edge luminance gradient, also referred to as the "bulls-eye effect", is due not only to the intentional design of the filter to concentrate the luminous output in the forward direction, but also to the unintentional decrease in thickness of the filter from the center toward the edges of the display window, due in large part to the shadowing effect of an upstanding peripheral sidewall or skirt, which extends rearward from the display window and joins the display window to the funnel-shaped portion of the CRT envelope.
This bulls-eye effect is partially alleviated by the use of the convexly curved inner surface of the display window, which tends to direct most of the light into the projection lens. However, since the radius of curvature of this inner surface is not an exact match for that of the projection lens, the bulls-eye effect cannot be completely compensated by this technique.